Property-ization of Methods

Typing ```inbound.to()``` felt wrong to me.

I changed such that the much cleaner ``inbound.to``` works instead.

Ultimately, we are interested in what strings the class returns, not that the methods are methods rather than instance variables.

When I was writing some attachment code, the absurdity of ```inbound.attachments()[0].name()``` convinced me to spend the 5 minutes for this fix. ```inbound.attachments.name``` is much cleaner, and, I'd argue, more Pythonic.

I updated README & Tests to reflect change.

Unfortunately, this is likely to break existing implementations on this module.